Distillation is the oldest and most commonly used method of desalination. The world's first land-based desalination plant, a multiple-effect distillation (MED) process plant that had a capacity of 60 m3/day, was installed on Curaçao, Netherlands Antilles, in 1928. Further commercial development of land-based seawater distillation units took place in the late 1950s, and initially relied on the technology developed for industrial evaporators (such as sugar concentrators) and for the shipboard distillation plants which were built during World War n. The multistage-flash (MSF), MED, and vapor-compression (VC) processes have led to the widespread use of distillation to desalinate seawater.

Technical Description

Distillation is a phase separation method whereby saline water is heated to produce water vapor, which is then condensed to produce freshwater. The various distillation processes used to produce potable water, including MSF, MED, VC, and waste-heat evaporators, all generally operate on the principle of reducing the vapor pressure of water within the unit to permit boiling to occur at lower temperatures, without the use of additional heat. Distillation units routinely use designs that conserve as much thermal energy as possible by interchanging the heat of condensation and heat of vaporization within the units. The major energy requirement in the distillation process thus becomes providing the heat for vaporization to the feedwater.

· Multistage Flash (MSF)

Figure 18 shows a simplified schematic of a multistage-flash unit. The incoming seawater passes through the heating stage(s) and is heated further in the heat recovery sections of each subsequent stage. After passing through the last heat recovery section, and before entering the first stage where flash-boiling (or flashing) occurs, the feedwater is further heated in the brine heater using externally supplied steam. This raises the feedwater to its highest temperature, after which it is passed through the various stages where flashing takes place. The vapor pressure in each of these stages is controlled so that the heated brine enters each chamber at the proper temperature and pressure (each lower than the preceding stage) to cause instantaneous and violent boiling/evaporation.

The freshwater is formed by condensation of the water vapor, which is collected at each stage and passed on from stage to stage in parallel with the brine. At each stage, the product water is also flash-boiled so that it can be cooled and the surplus heat recovered for preheating the feedwater.

Because of the large amount of flashing brine required in an MSF plant, a portion (50% to 75%) of the brine from the last stage is often mixed with the incoming feedwater, recirculated through the heat recovery sections of the brine heater, and flashed again through all of the subsequent stages. A facility of this type is often referred to as a "brine recycle" plant. This mode of operation reduces the amount of water-conditioning chemicals that must be added, and can significantly affect operating costs. On the other hand, it increases the salinity of the brine at the product end of the plant, raises the boiling point, and increases the danger of corrosion and scaling in the plant. In order to maintain a proper brine density in the system, a portion of the concentrated brine from the last stage is discharged to the ocean. The discharge flow rate is controlled by the brine concentration at the last stage.

· Multiple Effect (MED)

In multiple-effect units steam is condensed on one side of a tube wall while saline water is evaporated on the other side (in a manner similar to the VC process shown in Figure 19). The energy used for evaporation is the heat of condensation of the steam. Usually there is a series of condensation-evaporation processes taking place (each being an "effect"). The saline water is usually applied to the tubes in the form of a thin film so that it will evaporate easily. Although this is an older technology than the MSF process described above, it has not been extensively utilized for water production. However, a new type of low-temperature, horizontal-tube MED process has been successfully developed and used in the Caribbean. These plants appear to be very rugged, easy to operate, and economical, since they can be made of aluminum or other low-cost materials.

The vapor-compression process uses mechanical energy rather than direct heat as a source of thermal energy. Water vapor is drawn from the evaporation chamber by a compressor and except in the first stage is condensed on the outsides of tubes in the same chambers, as is shown in Figure 19. The heat of condensation is used to evaporate a film of saline water applied to the insides of the tubes within the evaporation chambers. These units are usually built with capacities of less than 100 m'/day and are often used at resorts and industrial sites.

Membrane distillation is a relatively new process, having been introduced commercially only in the last few years. The process works by using a specialized membrane which will pass water vapor but not liquid water. This membrane is placed over a moving stream of warm water, and as the water vapor passes through the membrane it is condensed on a second surface which is at a lower temperature than that of the feedwater.

· Dual Purpose

Most of the large distillation units in the world are dual-purpose facilities. Specifically, they derive their source of thermal energy from steam that has been used for other purposes, usually for power generation. Thus, the feedwater is heated in a boiler to a high energy level and passed through a steam turbine before the steam is extracted for use at a lower temperature to provide the heat required in the distillation plants. At this point, the desalination then conforms to the processes described above.

Extent of Use

Since 1971, about 65 single-purpose service or experimental plants have been installed in Latin America and the Caribbean, with capacities ranging from 15 to 1 000 m3/day. In Mexico they supply freshwater to fishing villages and/or tourist resorts in Baja California and in the north-central and southeastern parts of the country. They also provide freshwater to agricultural communities.

Desalination for municipal freshwater supply purposes started in Mexico in the late 1960s, when the Federal Electricity Commission installed two 14 000 m3/day MSF distillation units in its Rosarito Power Plant in the city of Tijuana in northwest Mexico. At that time, those units were among the largest in the world. The Federal Electricity Commission currently operates about 31 desalination plants to produce high-quality boiler make-up water, and maintains the two dual-purpose units in Tijuana. The Mexican Navy also installed some smaller solar distillation plants to provide a supply of freshwater to some islands in the Pacific Ocean. PEMEX, the national oil company of Mexico, operates about 62 small seawater desalination plants for human freshwater consumption on off-shore oil platforms or ships. These distillation units are mainly VC, waste heat, submerged-tube evaporators, and RO plants.

The island of Curaçao, in the Netherlands Antilles, currently has two distillation plants. One is for public water supply and the other is used by the oil refinery PEDEVESA. Both use the MSF process. The public supply plant has a maximum design capacity of 47 000 m3/day (although the average daily production is currently 41 000 m3/day), which is higher than the estimated domestic water consumption of 35 000 m3/day.

Operation and Maintenance

Most plants are installed in isolated locations where construction is troublesome and where the availability of fuel, chemicals, and spare parts is limited. In these places, there is usually also a scarcity of qualified personnel; therefore, people are often selected from the local communities and trained to operate the plants. The operation of distillation plants requires careful planning, well-trained operators, and adequate operation and maintenance budgets to guarantee the supply of good quality water. Except for an annual shutdown of 6 to 8 weeks for general inspection and maintenance, the operation of desalination plants is usually continuous. Maintenance and preventive maintenance work, for a MSF plant, consists of:

· Repairing damage (cracks) to the stainless steel liners in the stages.

· Removing scale and marine growths in the tubes in all stages using high pressure "hydrolaser" sprayers.

· Removing the vacuum system ejectors for cleaning, inspection, and replacement as necessary; most parts have a lifetime of 3 to 4 years.

· Inspecting all pumps and motors, replacing bearings and bushings, and renewing protective coatings on exposed parts (e.g., pumps must be primed and painted before being installed).

Level of Involvement

The manufacturing capacity to produce MSF evaporators is available in those places where power plant equipment is fabricated. Thus, many countries in Latin America have the potential to manufacture locally the equipment needed to develop desalination plants. Further, some local manufacturers have signed licensing agreements with major foreign desalination manufacturing firms as a result of governmental policies of import substitution, in order to offer desalination equipment, particularly MSF plants, to the electric-generating industry in the region.

In die Caribbean, desalination by distillation is being used primarily in the private sector, especially in the tourist industry. Some industrial concerns and power companies have incorporated distillation into their operations as part of a dual process approach. Government participation has been very limited. Future developments of this technology, which are expected to reduce the cost of desalination plants, will be likely to encourage greater government participation in the use of distillation in the development of public water supply systems.

Costs

The production cost of water is a function of the type of distillation process used, the plant capacity, the salinity in the feedwater (seawater or brackish water), and the level of familiarity with the distillation process that exists in the region. Table 7 shows a range of costs that have been reported by different countries using this technology. Production costs appear to increase in proportion to the capacity of the plant. In many applications, distillation provides the best means of achieving waters of high purity for industrial use: for volumes of less than 4 000 m'/day, the VC process is likely to be most effective; above that range, the MSF process will probably be preferable.

Desalination of seawater is a relatively expensive method of obtaining freshwater. The MSF system has proved to be a very efficient system, when properly maintained. It produces high quality product water (between 2 and 150 mg/1 of total dissolved solids at the plant in Curaçao); TDS contents of less than 10 mg/1 have been reported from the VC plant in Chile. Because the water is boiled, the risk of bacterial or pathogenic virus contamination of the product water is minimal.

Suitability

MSF plants have been extensively used in the Middle East, North Africa, and the Caribbean. Although MED is an older technology than the MSF process, having been used in sugar refineries, it has not been extensively utilized for water production. However, the new low-temperature horizontal-tube MED process has been successfully used in the Caribbean, usually in units with capacities of less than 100 m'/d (25,000 gpd) installed at resorts and industrial sites.

· Many plants are fully automated and require a limited number of personnel to operate.

· Distillation has minimal environmental impacts, although brine disposal must be considered in the plant design.

· The technology produces high-quality water, in some cases having less than 10 mg/1 of total dissolved solids.

· Distillation can be combined with other processes, such as using heat energy from an electric-power generation plant.

Disadvantages

· Some distillation processes are energy-intensive, particularly the large-capacity plants. «Disposal of the brine is a problem in many regions.

· The distillation process, particularly MSF distillation, is very costly.

· Distillation requires a high level of technical knowledge to design and operate.

· The technology requires the use of chemical products, such as acids, that need special handling.

Cultural Acceptability

Despite significant progress toward becoming more energy-efficient and cost-effective, the level of community acceptance of distillation technologies is still limited. Their use is mainly restricted to resort hotels and high-value-added industries, and to the Caribbean islands.

Further Development of the Technology

Research into the falling (or spray) film MED thermal desalination process suggests that further development of distillation technologies can produce product waters that are comparable in quality to those produced with current MSF technologies and also offer additional advantages, including lower pumping requirements, higher heat transfer rates, and greatly reduced pressure differentials across the heat transfer surfaces. These favorable comparisons also apply to a falling (or spray) film VC design. Some additional considerations include:

· Improved materials for evaporator heat transfer surfaces (aluminum has two major benefits over other materials: a lower cost than copper-nickel, with nearly triple the thermal conductivity and higher operating temperatures, with an upper limit of 150° F (63" C) for aluminum alloys containing approximately 2% magnesium).

· Improved coatings for use in shell construction (with aluminum evaporator heat transfer surfaces, it is essential to prevent corrosion caused by the proximity of other metal ions; the carbon steel shell must be appropriately coated, and provision made for all supporting structures to be protected).

· Improved piping material for use with low temperature distillation techniques; piping should be of PVC, fiberglass, or other suitable non-metallic material.

A further alternative and promising new concept for a dual purpose plant has been the development of an evaporative condenser which is equipped with dimpled flat plate elements that could greatly increase the efficiency of this type of plant.